A number of techniques presently exist for extending the time over which fruit and vegetables can be successfully stored without seriously affecting their quality between harvest and consumption. Such storage techniques are used to preserve various crops during transportation from one part of the world to another and to make seasonal commodities available to the consumer during other parts of the year.
Fresh fruits and vegetables are living tissues which continue to respire after harvesting. The process of respiration involves the use of oxygen in breaking down the food reserve contained within the fruit or vegetable, releasing energy and producing carbon dioxide. The rate of respiration, and therefore the rate of loss of the food reserve and deterioration of the commodity, is closely related to the respiration rate.
To prolong the storage periods of fruits and vegetables, their respiration rate is reduced by lowering the temperature and oxygen levels of the environment in which they are stored and by allowing the carbon dioxide level to increase. However, lowering the temperature too far will cause damage by freezing or chilling injury. Reducing the oxygen concentration too much will cause fermentation to occur within the fruit or vegetable which accelerates the ageing process and possibly causes other forms of damage associated with low oxygen levels. A storage environment containing excessive concentrations of CO2 can also cause damage to fruit and vegetables. Damage resulting from incorrect environmental storage conditions reduces the quality and market potential of the produce.
The precise level of temperature, oxygen and carbon dioxide required to maximize storage life and to minimize storage disorders varies widely, depending on the type of produce, cultivars, growing conditions, maturity, harvest conditions, and post-harvest treatments. The ideal storage conditions can also depend upon where the particular product is grown and can vary from season to season. Recommended levels for different kinds of produce, which may be based, for example, on a crop's storage behaviour in previous years, are published by various national research bodies and extension advisors, and are considered to be the best compromise between extending life and minimizing storage disorders. The storage facilities are controlled to maintain the storage environment for a particular product at these recommended fixed levels. Because of the number of factors and their variability on which the ideal storage conditions depend, maintaining the product at the recommended levels may result in premature damage, in which case storage of the product has to be curtailed or loss is incurred. On the other hand, as the recommended levels often include a safety margin above a known damage threshold, the respiration rate of the produce is necessarily above the minimum the produce can tolerate, possibly leading to a shortened storage time.
A system for controlling the air composition in a room for storing vegetable products is disclosed in International Patent Application, Publication No. WO-A-96/18306. In one example, the system includes carbon dioxide and oxygen sensors for sensing the carbon dioxide and oxygen content, respectively, of a storage room in which vegetable products are stored. Under the control of a computer processor, the oxygen level in the storage room is reduced and the ratio between the carbon dioxide and oxygen levels is monitored. For normal respiration, the amount of carbon dioxide produced by the stored product is approximately equal to the oxygen consumed by the product so that the ratio of carbon dioxide to oxygen should be and remain equal to approximately 1, as the oxygen level is reduced. If the oxygen level is decreased too far, fermentation occurs where no oxygen is consumed but carbon dioxide is still produced, in which case the ratio of carbon dioxide to oxygen becomes greater than 1. The control system reduces the oxygen content until the latter condition is observed and thereafter increases the oxygen content slightly. If the ratio returns to 1, the oxygen content is again lowered until an increase in the ratio is detected. In another example, the occurrence of fermentation in the stored vegetable product is detected directly by measuring the presence of metabolites such as ethanol or lactate, formed by the fermentation process. In this case, the oxygen content is lowered until the presence of ethanol or lactate in the storage room is detected by a sensor and thereafter the oxygen content is slightly increased. If the increase is sufficient to bring the ethanol or lactate levels down to an unmeasurable level, the oxygen content is again gradually decreased until a measurable amount of lactate or ethanol is detected.
A method of testing the post-harvest quality of fruits and vegetables, such as firmness, texture, aroma and color using chlorophyll fluorescence is disclosed in U.S. Pat. No. 5,822,068. The method involves irradiating a fruit or vegetable sample firstly with low level red light to stimulate minimal fluorescence within the chlorophyll and detecting the intensity of the minimal fluorescence, Fo, emitted by the sample, and shortly thereafter irradiating the sample with high level red light to stimulate maximum fluorescence within the chlorophyll and detecting the maximal fluorescence intensity, Fm, emitted by the sample. A relatively high value of either of these signals is taken as an indication of good quality, whereas lower values in the fluorescence signals are correlated to lower quality in the product.
Chlorophyll fluorescence techniques have also been used to detect damage and disorders in apples caused by low oxygen levels. One such study is described in: The Proceedings of the 7th Controlled Atmosphere Conference, Volume 2, pp 57–64 (1997), “Chlorophyll fluorescence detects low oxygen stress in “Elstar” apples”, R. K Prange, S. P. Schouten and O. van Kooten, in which the minimal fluorescence intensity signal Of and the ratio (Fm−Fo)/Fm were measured for Elstar apples stored over a period of 20 days in an atmosphere containing 0.07% oxygen. The results show that Fo increased over the test period whereas (Fm−Fo)/Fm decreased. Independent quality measurements indicated that some of the low oxygen treated samples were firmer than the control samples, which were stored in air, and that the only disorder observed in the low oxygen treated apples was a gradual increase in an off-flavour during the 20 day treatment period.